Polymeric microparticles are used in applications of many kinds as a solid phase to bind biomaterial. Immunoassays, DNA hybridisation, PCR, separation of cells, separation of proteins and cleaning of proteins can be mentioned as examples of such applications.
A large solid phase surface area and short diffusion distances are advantageous features of microparticles.
The size of microparticles is usually in the range 0.05-10 .mu.m. They are available in different materials and pre-activated for many applications. The microparticles are separated from the solution in a centrifugal or filtration process.
Magnetic microparticles are also widely used. Their advantage is that they can be separated from the solution by using an outside magnet, whereby no centrifugal or filtration process is required.
In presently used processes for separating magnetic particles, the reaction vessel is kept in a magnetic field so that the particles gather together and form a so-called pellet on the bottom of the vessel. The solution free of particles is then removed by decantation or suction. The separation of magnetic particles is more simple, quicker and gentler than the separation of conventional particles. However, the solution must be removed very cautiously from the vessel, so that no particles are removed at the same time.
In publication U.S. Pat. No. 4,272,510 magnetic pins, on the tip of which the particles will adhere, are proposed for use for separating macrosized (about 0.1-20 mm) magnetic particles in immunoassays. The particles are removed mechanically from the pins by pushing or by pulling with the aid of stronger magnets located under the vessel.
In publication EP-140787 a method has been proposed, wherein microsized magnetic particles are separated from a solution with the aid of a magnetic rod which is Immersed in the solution. The particles are pulled off the rod by using a more powerful magnet.
In publication WO-86/06493 a method has been proposed for use in immunoassays, wherein magnetic particles and an adhered labelled complex are separated from the solution by using a magnetic rod and thereafter removed for measurement. The rod has a fixed magnet and the tip of the rod has a removable protective cover, on the outer side of which the particles adhere. After the separation and before the measurement the protective cover is preferably covered with another protective cover. After the measurement, the covers together with the particles are removed and disposed of and they are replaced-with new protective covers for a new separation. According to the publication, the magnet may also be an electromagnet, in which case the magnetic field can be removed when desired.
In publication WO-87/05536, in turn, a rod provided inside with a vertically movable magnet has been proposed for separating magnetic particles. With the magnet in the bottom position the rod is immersed in a solution containing particles, whereby the particles will gather onto the end of the rod. By allowing the magnet to move to the top position the particles can drop off the rod. In this way particles can be collected and moved from one solution to another.